The 3.75 you can use a ground stock 440 crank - in the usa much cheaper than a new one

4.25 needs the block cleaned up internally to stop bits a pieces hitting things - in a "B" block it -generally- means external oiling too

there are concepts of bigger valves needed for the flow capacity and valve shrouding on the smaller bore too, many dont sweat that stuff too much though

Off the line @1200... look into your traction control before adding more power, cause it aint going to do you any good if it cant hook up